The truth is that we destroyed the institutions of authority in Iraq without having the foggiest idea what would come next. As one senior British general has put it to me, “we snipped the spinal cord” without any plan to replace it. There are more than 100,000 dead Iraqis who would be alive today if we had not gone in and created the conditions for such a conflict, to say nothing of the troops from America, Britain and other countries who have lost their lives in the shambles.

That is the truth, and it is time Tony Blair accepted it. When we voted for that war – and I did, too – we did so with what now looks like the hopelessly naive assumption that the British and American governments had a plan for the aftermath; that there was a government waiting in the wings; that civic institutions would be preserved and carried on in the post-Saddam era.

In other words, I wanted to get rid of Saddam, and I fondly imagined that there would be a plan for the transition – as there was, say, with Germany in 1945, where the basic and essential machinery of government was continued, despite the programme of de-Nazification. I felt so nervous (and so guilty) about this assumption, that I went to Baghdad in the week after the fall of Saddam, to see if I was right. I was not.

I remember vividly the mystification on the face of a tall, grey-haired CIA man in his fifties, wearing a helmet and body armour, whom I found in one of the government ministries. He and I were alone among a thousand empty offices. The entire civil service had fled; the army was disintegrated.

He was hoping to find someone to carry on the business of government – law and order, infrastructure, tax collection, that kind of thing. The days were passing; the city was being looted; no one was showing up for work. We had utterly blitzed the power centres of Iraq with no credible plan for the next stage – and frankly, yes, I do blame Bush and Blair for their unbelievable arrogance in thinking it would work.

As time has gone by, I am afraid I have become more and more cynical about the venture. It looks to me as though the Americans were motivated by a general strategic desire to control one of the biggest oil exporters in the world, as well as to remove Saddam, an unpleasant pest who had earlier attempted to murder the elder Bush. Blair went in fundamentally because he (rightly) thought it was in Britain’s long-term interest to be closely allied with America, and also, alas, because he instinctively understood how war helps to magnify a politician. War gives leaders a grandeur that they might not otherwise possess. If you hanker after Churchillian or Thatcherian charisma, there is nothing like a victorious war.

The Iraq war was a tragic mistake; and by refusing to accept this, Blair is now undermining the very cause he advocates – the possibility of serious and effective intervention. Blair’s argument (if that is the word for his chain of bonkers assertions) is that we were right in 2003, and that we would be right to intervene again.

Many rightly recoil from that logic. It is surely obvious that the 2003 invasion was a misbegotten folly. But that does not necessarily mean – as many are now concluding – that all intervention is always and everywhere wrong in principle, and that we should avoid foreign entanglements of all kinds.

Yes, we helped cause the disaster in Iraq; but that does not mean we are incapable of trying to make some amends. It might be that there are specific and targeted things we could do – and, morally, perhaps should do – to help protect the people of Iraq from terrorism (to say nothing of Syria, where 100,000 people have died in the past three years).

Britain is still a power on the UN security council. We spend £34 billion a year on defence. We have fantastic Armed Services full of young, optimistic and confident men and women who are doing a lot of good – in spite of the cotton-wool legislation that now surrounds them – in dangerous places across the world.

It would be wrong and self-defeating to conclude that because we were wrong over Iraq, we must always be wrong to try to make the world a better place. But we cannot make this case – for an active Britain that is engaged with the world – unless we are at least honest about our failures.

Somebody needs to get on to Tony Blair and tell him to put a sock in it – or at least to accept the reality of the disaster he helped to engender. Then he might be worth hearing. The truth shall set you free, Tony.